Disposable transducer seal

ABSTRACT

A transducer seal designed to seal an open aperture of a transducer housing for a therapeutic ultrasound procedure. The seal has a membrane, a retainer and a mating device for locking in place with the transducer housing. The membrane is essentially transparent to ultrasound energy while being stretched taut about the retainer. The transducer seal may be made in a disposable or re-usable form.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.11/935,481 (Attorney Docket No. 021356-000720US), filed Nov. 6, 2007,which is a continuation of U.S. application Ser. No. 11/027,491(Attorney Docket No. 021356-000710US), filed Dec. 29, 2004, which is acontinuation-in-part of U.S. patent application Ser. No. 10/750,369(Attorney Docket No. 021356-000700US), filed Dec. 30, 2003 (nowabandoned) entitled “Disposable Transducer Seal”, and related to that ofthe following applications each of which was filed on 30 Dec. 2003: Ser.No. 10/750,370, entitled “Medical Device Inline Degasser” (nowabandoned); Ser. No. 10/751,344, entitled “Articulating Arm for MedicalProcedures”; 60/533,528, entitled “Position Tracking Device”;60/534,036, entitled “Ultrasound Therapy Head with Movement Control”;60/533,958, entitled “Systems and Methods for the Destruction of AdiposeTissue”; 60/534,034, entitled “Component Ultrasound Transducer”; thefull disclosure of each of these applications are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention pertains to a sealing device for retaining acoupling fluid such as degassed water within an ultrasound transducerhousing.

2. Description of the Prior Art

Ultrasound transducers require a coupling medium to connect thetransducer to a patient in order to minimize the reflection andrefraction of ultrasound waves when those waves cross a border betweentwo materials of similar acoustic properties. One of the biggest issuesin coupling transducers to a patient either for a diagnostic ultrasounddevice, or a therapeutic ultrasound device, is the presence of air.Coupling agents are used to eliminate air bubbles between the transducerand the patient. For diagnostic purposes, mineral oils, hydro-gels andeven water can be used to couple a transducer to a patient. Intherapeutic procedures the coupling agent should be more strictlycontrolled so that even minute air bubbles are eliminated.

In high intensity focused ultrasound (HIFU) procedures the need tocouple the transducer to the patient often includes a means of coolingthe face of the transducer, or cooling a patient's skin, with a mediumthat will pass ultrasound energy with little or no attenuation oradverse effect. Typically this medium is water, sometimes withantibacterial additives, held within a transmission cavity with a cap ormembrane, and through which the ultrasound energy passes.

One major issue with such a system arises from bubble formation causedby dissolved gasses being drawn out of solution. These bubbles providean impedance mismatch to the ultrasound energy, causing scattering andlocalized heating, leading to observed effects such as reducedeffectiveness of therapy, the destruction of the cap or seal, or patienthyperemia.

Atmospheric water for example, contains approximately 8.5 PPM (parts permillion) O₂, and 14.5 PPM N₂ as well as other dissolved gasses. Usingdissolved oxygen (DO) as an indicator, it is possible to determine therelative contents of other gasses, CO₂, CO, N₂, etc . . . This can bedone using the partial pressure values of the other gases. Reducing theconcentration of DO (and other gases) inhibits the incidence ofcavitation. However for high intensity focused ultrasound (HIFU)procedures, the optimal dissolved gas content is highly dependent on thetreatment being performed, and the type of ultrasound instruments beingutilized. To date, we are aware of no treatise clearly defining theoperable boundaries of DO and other dissolved gases in HIFU operations.

The common method used by the industry is to prepare the fluid bypassing it through a filtration and de-ionization process to removeimpurities and particulates that may precipitate out, contaminate orprovide nucleation sites for bubbles. The coupling fluid is thendegassed to some minimum level before introduced into the system.Typically degassing is performed by bulk cavitation under a vacuum orboiling at atmospheric or sub atmospheric pressure and then sealing thedegassed fluid in a container.

In a completely sealed system the dissolved gas content will remainconstant, but as described below the gas content will strive to meetequilibrium with the partial pressure of the local atmosphericconditions. During short procedures or low power ultrasound proceduresthe re-gas rate is usually slow enough not to cause problems. In longerprocedures and/or at higher powers, the probability that re-dissolvedgas will be drawn into the fluid, and subsequently interfere withultrasound transmission, goes up considerably since it is impossible toprevent gas diffusing through the system lining, joints and sealswithout investing in prohibitively expensive parts and materials.

The methods by which gasses come out of solution or enter the coolingsystem are various, some examples of the more common range from pressurechanges within the cooling system caused by physical restriction toatmospheric conditions. Local pressure changes such as rectifieddiffusion from HIFU or temperature changes will bring gas out ofsolution as will displacement of the partial pressure of one gas byanother, or by material leaching. Other methods by which gas may enterthe system include diffusion through the tubing, seals and structure ofthe cooling system in the same way a balloon deflates, trapping microbubbles within the surface structure and pockets of the cooling system,chemical reactions between materials in the cooling system, or as a byproduct of bacterial growth within the cooling system.

Precautions such as using low permeability materials for the tubing areregularly employed, but even with such precautions, the re-gas rate canbecome a major issue. Other methods used to reduce the effects ofre-gassing include the introduction of surfactants or wetting agents toprevent bubble formation, using larger volumes of fluids, and the use ofhydrophilic and/or hydrophobic polymers such as polyvinylpyrolidone(PVP). Experimental testing has shown these provide only a short termsolution.

Numerous examples in the prior art show differing solutions to theproblems of dealing with coupling HIFU transducers to a patient as wellas providing an apparatus for degassing a fluid. However there has beenthus far nothing demonstrating the feasibility or utility of an in linedegassing mechanism combined with a HIFU therapy system during an actualmedical procedure or application. The use of an inline degasser during aprocedure mandates the use of a transducer housing having a cavity wherethe cooling/coupling fluid may circulate around the transducer. Toprevent the coupling fluid from escaping the cavity and to minimize gasfrom entering the cavity, a seal is needed.

The inability of the prior art to maintain a controlled dissolved gascontent in a cooling fluid over a prolonged procedure acts as a forcedlimitation to prolonged HIFU therapy.

Thus there remains a need for a seal capable of retaining a degassedcoupling fluid for use in a HIFU procedure within a cavity containing aHIFU transducer.

BRIEF SUMMARY OF THE INVENTION

It is an objective of the present invention to provide for a seal thatis both inexpensive to manufacture, and that can be quickly and easilyinstalled into a transducer housing.

It is a further object of the invention to make a seal that isdisposable so reuse and re- sterilization concerns can be avoided.

These objectives are provided for in a disposable transducer sealcomprising a membrane being substantially transparent to ultrasoundenergy, the membrane being non-porous to water and acoustic couplingfluids; a retainer having a substantially annular configuration forholding the membrane, and a means for mating the retainer with atransducer housing.

In an alternative embodiment, there is an apparatus for maintaining abarrier between a transducer housing and an outside environment. Theapparatus comprising a membrane being substantially transparent toultrasound energy and a means for sealing the membrane to the transducerhousing wherein the sealing means provides for a substantially fluidtight barrier between the transducer housing and the outsideenvironment.\

In another embodiment there is a method of preparing an ultrasoundtransducer housing having a coupling fluid chamber for use. The methodcomprising the steps of (a) preparing an ultrasound transducer housingfor receiving a coupling fluid; (b) engaging a transducer sealing deviceto the ultrasound transducer housing such that the coupling fluidchamber is sealed, and (c) filling the chamber with a coupling fluid.

There is still further a method of making a transducer sealing device,the method comprising the steps of: (a) forming a retainer having anengaging fitting element; (b) placing an acoustically transparentmaterial across the retainer; and (c) securing the material to theretainer.

There is still another method of sealing a transducer housing using atransducer sealing device, the method comprising the steps of: (a)forming an acoustically transparent material into a shape for fittinglyengaging a transducer housing having a fluid chamber; and (b) securingthe shape to the transducer housing such that the fluid chamber issubstantially sealed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-C show three configurations of a transducer sealing device.

FIGS. 2A-C show cross-section views of the transducer sealing device.

FIG. 3 illustrates a cut away view of a transducer sealing device on atransducer.

DETAILED DESCRIPTION OF THE INVENTION

The device of the present invention is a disposable transducer seal(seal). The seal is designed for use with a HIFU therapy system forcosmetic applications, such as the reduction or ablation of adiposetissue. The seal comprises a membrane, a retainer and a means forattaching the seal to a transducer housing. The transducer housing isshaped similar to an inverted cup having a gap space for a couplingfluid such as degassed water. The seal is used to retain the degassedwater in the gap space without the water spilling onto a patient duringan ultrasound procedure. The seal is intended to provide both an airtight seal, and a barrier to prevent cross contamination of thedifferent fluids on opposite sides of the membrane.

More specifically, in one embodiment, there is a disposable transducerseal comprising a membrane being substantially transparent to ultrasoundenergy, the membrane being non-porous to water and acoustic couplingfluids. A retainer having a substantially annular configuration is usedfor holding the membrane, and there is a means for mating the retainerwith a transducer housing.

In another embodiment, there is an apparatus for maintaining a barrierbetween a transducer housing and an outside environment. The apparatuscomprising a membrane being substantially transparent to ultrasoundenergy, and a means for sealing the membrane to the transducer housingwherein the sealing means provides for a substantially fluid tightbarrier between the transducer housing and the outside environment.

The membrane used must be suitable for use with an ultrasoundtransducer, while simultaneously providing a non-porous barrier betweenthe coupling fluid used within a transducer housing, and the environmentoutside. Numerous materials are usable for a membrane, and in general wehave found the membrane properties must provide the necessary level ofacoustic transparency combined with the ability to prevent air diffusioninto the coupling fluid. The presence of dissolved gasses in a couplingfluid can adversely affect HIFU therapy.

Standing water may contain high levels of dissolved gases. We measureddissolved oxygen (DO) content of tap water left to stand at roomtemperature for several hours and found levels that produced high levelsof cavitation when used as a coupling fluid for a HIFU ultrasoundsystem. By degassing the coupling fluid, the incidence of cavitation canbe greatly reduced. We have found that when DO levels are reduced below5 parts per million (PPM), the incidence of cavitation is greatlyreduced. Furthermore we have found that by chilling the coupling fluid,the incidence of cavitation can be further controlled. Optimally we haveobserved that where DO levels drop to less than 2 PPM, cavitation, alongwith its detrimental effects (such as hyperemia, the burning of thepatient skin due to energy focused in the coupling fluid instead of theultrasound transducer focal zone), are almost completely absent from thecoupling fluid.

Therefore it is highly desirable that the membrane have sufficientintegrity to prevent the diffusion of gasses across the membrane, suchthat the coupling fluid maintains a DO content of 5 PPM or less. Themembrane may be somewhat porous to gas if there is a degassing systemused in conjunction with the coupling fluid. However, it would still benecessary for the membrane to be sufficiently robust to prevent gasdiffusion into the coupling solution at a rate faster than the degassingsystem can remove it.

The membrane is composed of a compound or material being essentially orsubstantially transparent to ultrasound energy. Acoustic transparencycan be determined for a material through computer simulation orexperimentation. We measured intensity levels dropping 1 dB or less withthe introduction of a 50 micron thick polyimide membrane. The membranemay be composed of naturally occurring materials such as latex rubber,or a synthetic material like a thin film plastic or synthetic rubber.Uniformity in the membrane of the seal is desirable as it reduces thedetrimental effects on the ultrasound beam during a procedure. Athermoforming polyimide can readily be formed into a desired shape andprovides a good example of a material that can be used for a membrane.For manufacturing considerations and for optimal performance, thesynthetic polyimide is preferred. The membrane may be flexible orinflexible as long as it is drawn taut about the retainer. (While themembrane may be inflexible, it is preferred the membrane be a littleflexible so that it can conform to the curves of a patients body morereadily.) Some flexibility also allows the membrane to respond to fluidpressure changes during procedures. This responsiveness during aprocedure helps maintain a constant pressure environment for the fluid,since the membrane may expand a little or contract a little due tovariations in pressure in the system. The membrane serves as an acousticwindow, so it is desirable that the membrane is substantiallytransparent to ultrasound energy. Smoothness in its surfaces duringmanufacturing will help reduce signal scattering, reflection orattenuation, thus improving performance of the membrane. A membranehaving desirable acoustic properties is required. If the membrane isthermo-formable, it allows for easier manufacturing of the membrane. Themembrane is preferably made a uniform thickness, so as to reduce signalscattering or other loss of the ultrasound signal passing through themembrane. The membrane described above is merely illustrative and manyother possible materials and methods could be used without departingfrom the spirit of the present invention.

The retainer may be assembled from any material. However since theretainer may be in direct contact with the patient, it is preferred tobe made of a material that is easily formable (such as an extrudedplastic, or moldable plastic) so that the sealing device may bediscarded after a single use. The membrane is drawn taut over theretainer, or drawn taut and the retainer is placed down about themembrane so that the membrane remains taut during a medical procedure.Alternatively the membrane may be made of a material highly resistant tocleaning solutions (e.g. disinfectant solvents) or cleaning procedures(e.g. autoclave), so the transducer seal may be cleaned and reused anumber of times. However because high intensity ultrasound has adverseeffects on materials used in the membrane component of the transducerseal, ultimately a material having high acoustic transparency also lackssufficient stability to be “permanent” or “non-disposable” device, andafter a limited number of uses, even a resistant or robust transducersealing device, will necessarily be discarded.

The seal has a means for mating with a transducer housing. The means maybe such as the retainer is shaped as an interlocking ring with thetransducer housing having a conforming receiving aperture or shape. Orthe transducer housing may have clips for latching on to tabs on theretainer. Other means of mating to the transducer include a magneticlock, a screw-in pin, a temporary adhesive, an interference fitting maleand female part (one being on the retainer, the corresponding part onthe transducer housing). The principle concern is to reduce the leakageof fluid and gas across the membrane and/or the retainer to preserve theoperating environment of the transducer.

The retainer may also include a means for identifying the sealing deviceto the transducer housing, or its attached ultrasound system. The meansmay be an electronic device such as an encoded chip or flex circuit, orit may be linked to the mating means, such that if the mating is notproperly done the transducer housing and corresponding ultrasound systemwill not recognize the retainer and therefore remain in a safe mode. Useof an electronic device further provides for the means to encodeadditional information into the chip or circuit. For instance the lifeof the membrane may be such that it should not be used for more than acertain about of time, or for more than a certain number of active HIFUtransmissions. The circuit or chip may have a digital counter that canbe read, or written to, by the therapy head. In this case the membranecan be programmed with a “usable life” which if exceeded, would causethe chip to no longer be recognized by the therapy head or main system,and require the user to change the membrane or retainer for safetypurposes.

The recognition means may be a visually recognizable color code formatching colors and/or patterns of the transducer sealing device to atherapy head. The means may be a unique type of mechanical connection assimple as a customized thread of the retainer and therapy head screwconnection, an impedance detecting circuit or a safety device thatprohibits the therapy head from operating without the proper retainer inplace. The detection means may also be imprinted into the membranecomponent in the form of a chemically or optically detectable dopant.

Either the membrane or the retainer may also have a clear window. Thewindow is a small gap space designed to correspond to the location of anoptical emitter and photo-optical receiver such that an acoustic gelhaving a safety dopant can be detected by the transducer housing orultrasound system through the sealing device.

Although the principle embodiment is for the entire transducer sealingdevice to be disposable to facilitate the quick changing of the seal,other variations and reuse combinations are well within the spirit ofthe present invention. If a membrane material can be cleaned and reusedindefinitely, it would not be necessary to discard the membrane aftereach use. However during the course of our experimentation and computermodeling, we have not found such a material. We have found membranematerials that may be reused due to their resilience to cleaningsolutions and high intensity focused ultrasound energies. But ultimatelythese materials break down, lose uniformity of their surfaces, andbecome unacceptably porous to gas and fluid.

One variation is to provide either a shaped or flat membrane that may behandled by an operator as an individual component which is held in placeover a transducer housing using a reusable sealing means. Such means maybe a screw like cap, a snap fitting piece, an interference fittingpiece, O-ring or even the stretch tension of the membrane itself,assuming it is precisely shaped to the transducer housing. Any manner ofremovably joining the membrane to the housing would be acceptable.

The sealing device may be made using a variety of methods. One exampleis to form a retainer having an engaging fitting element for securingitself to the transducer housing. An acoustically transparent materialis placed across the retainer and then secured to it. The retainer has awindow or aperture through which acoustic energy can pass.

A second example is to form an acoustically transparent material into ashape for fittingly engaging the transducer housing and then securingthe shape to the housing.

The transducer housing can be prepared for receiving the sealing devicein numerous manners. The preferred embodiment of preparing thetransducer housing is to make sure the housing is ready to receive a newsealing device. If there is an old sealing device in place, the fluidshould be drained from it, then the housing can be inverted so thesealing aperture faces up. An old seal may be removed and a new oneplaced on the housing. If the housing is already clean and dry, then thesealing device need only be placed over the acoustic window and securedin place.

Referring now to the drawings, FIG. 1 illustrates several possibledesigns. The retainer 592 of the disposable transducer seal 590 has anannular configuration. The membrane 594 is drawn tightly around theretainer 592. Regardless of the material construction of the membrane,it is necessary for the membrane to be drawn tightly about the retainerand held in place. Thus if the membrane is a polymer formed into a thinlayer, or a softer latex rubber, the retainer serves to maintain theshape and rigidity of the membrane during use. If the membrane is asofter material, such as a latex rubber, then the retainer serves tokeep the membrane taut. Preferably the membrane has no slack in it, sothere is no play or deformation of the membrane during use. A limitedamount of deformity is desirable so the membrane can flex slightly to beconcave or convex relative to the transducer. However ripples in themembrane material, folds or even a somewhat flimsy shape to the membranemay have adverse effects on the transmission of ultrasound energy duringa procedure. The configuration is a circular ring, square, rectangle orother shape as may provide the best matching of the therapy head mouthto the transducer sealing device. Thus the annular configuration dependson the aperture of the transducer housing the seal must mate with. Theprecise shape will vary from one transducer housing to another. Theshapes shown are merely illustrative and not to be taken as limiting inany sense. Many shapes are possible, but the preferred embodiment is asubstantially annular ring.

FIG. 2 illustrates a cross section of the sealing device. As can be seenthe membrane is drawn tightly either within the retainer (FIG. 2A) oracross the surface face of the retainer (2B). Optional elements includethe encoder chip 596 illustrated in both drawings. FIG. 2C illustratesthe transducer head with a membrane 594 held in place by a pair ofthreaded retainers 592 a, 592 b. An O-ring 598 between the membrane andthe transducer housing 500 provides a fluid seal between the membraneand the transducer housing. Note: the drawing is not to scale, there areno gap spaces between the parts as shown in the illustration.

FIG. 3 illustrates the mating of the seal 590 to a transducer housing500. The housing is shaped similar to an inverted cup containing anelectronics and motor assembly for moving and controlling the transducerand any additional electronic components that may be integrated into thehousing. The seal 590 is placed over an open aperture on the transducerhousing. The design of the transducer housing is such that thetransducer is placed aperture end toward the patient, and the transducercan abut the skin of the patient. The seal is needed to prevent thedegassed water from leaking out, and to prevent air from leaking in.

The seal is mated to the transducer housing. The mating means may be anynumber of mechanical connections that allow for the air and water tightseal described above. Once the seal is in place, the cavity in thetransducer housing may be flooded with a coupling fluid without thefluid escaping. The seal may also have an electronic or mechanicalrecognition device such that the transducer housing will recognize theproper placement of the seal and move the ultrasound machine from a safemode to an active mode. Furthermore an optical window may be placedeither in the membrane or in the retainer so that any kind of opticalsensor or safety device using an optical sensor may still detect theproper safety material across the seal. Alternatively the membraneitself is optically transparent, or transparent to selected wavelengthsof visible light.

While the invention is susceptible to various modifications, andalternative forms, specific examples thereof have been shown in thedrawings and are herein described in detail. It should be understood,however, that the invention is not to be limited to the particular formsor methods disclosed, but to the contrary, the invention is to cover allmodifications, equivalents and alternatives falling within the scope ofthe appended claims.

1. A disposable transducer seal comprising: a membrane beingsubstantially transparent to ultrasound energy, said membrane beingnon-porous to water and acoustic coupling fluids; a retainer having asubstantially annular configuration for holding said membrane; and ameans for mating said retainer with a transducer housing.